Bipedalism and orthograde posture afford a number of selective advantages that eventually allowed ho

1
Backpedaling in human evolution Adaptive values
of bipedalism in reverse
J.K. McKee, A. Kolatorowicz, L. Reitsema, A.
Ruth, S. Schlecht, O. Ulvi, T. Weston
Department of Anthropology
It is our contention that the extensor muscles
would have evolved their full biomechanical
advantage with the attainment of modern body size
and proportions in Homo erectus, as scavenging
and hunting niches became a more critical in the
hominin dietary niche. This is consistent with
the Bramble Lieberman (2004) hypothesis
regarding the evolution of endurance running in
Homo. Moreover, it is at that stage that the
vestibular system allowed for backpedaling with
adequate agility (Spoor et al. 1994).
Bipedalism and orthograde posture afford a
number of selective advantages that eventually
allowed hominins to enter new niches. One
adaptive benefit that has eluded the literature
is the unique way in which bipedalism allows
hominins to move adroitly in reverse. Whereas
most mammals backpedal, few if any can do so with
comparable quickness and agility. We hypothesize
that rapid reverse motion may have provided a
small but significant advantage for Homo erectus
and later hominins.
A
A
B
B
Backward running, also known as
retro-running, is important in many sports,
particularly soccer, as one can move quickly
across the field while keeping the focused on
play action (Figure 1.) Speeds reached in
backward running can vary from 60-80 the speed
of forward running (Arata, 2000.) Backward
running is also used in training, as it uses most
of the same muscles as forward running, yet has
less impact on the skeletal system. Moreover, it
has been found to facilitate balance an
proprioception.
We conclude that rapid backpedaling would not
have provided a selective advantage at the
origins of bipedalism, but the morphological
exaptations that allow such behavior would have
had adaptive value for those who employed and
mastered appropriate technique.
Figure 2. From left to right, KNM ER 1481
(Homo), Al 288 (Australopithecus), AL 288
outlined Al 288 size-adjusted and superimposed
on KNM ER 1481. Homo is more human-like in
positioning of vastus lateralis and medialis. At
point A, there is a distal extension of the
greater trochanter interrupting the positioning
of vastus lateralis. Point B is where minor
lipping of linea aspera begins much more distally
in Australopithecus.
Vastus lateralis and vastus medialis are used
differentially in backward than forward running
(Flynn Soutas-Little, 1993) due to increased
effort for extension of the leg at the knee.
Observations of fossil femora of Australopithecus
afarensis, early Homo, and H. sapiens, reveal
that the proximal attachments of vastus lateralis
would necessarily differ in Australopithecus
(Figure 2). The lack of a pronounced linea
aspera on the proximal half of the femur also
suggests that both muscles would have been less
developed in the early hominins. Finally, as
with Neandertals (Trinkaus, 1983), the shorter
load arm (tibia) for the extensors in A.
afaraensis would would decrease speed relative to
Homo.
References Arata, A.W. 2000. Kinematic
comparison of high speed backward and forward
running. http//darkwing.uoregon.edu/btbates/back
ward/alan2.htm Bramble, D.M., and D.E. Lieberman.
2004. Endurance running and the evolution of
Homo. Nature 432 345-352. Flynn, T.W., and R.W.
Soutas-Little. 1993. Mechanical power and
muscle action during forward and backward
running. J. Orthop. Sports Phys. Ther. 17(2)
108-112. Spoor, F., B. Wood, and F. Zonneveld.
1994. Implications of early hominid labyrinthine
morphology for evolution of human bipedal
locomotion. Nature 369 645-648. Trinkaus, E.
1983. The Shanidar Neanderthals. New York
Academic Press.
Figure 1. Backward running is recognizable by
the upright trunk and greater extension of the
leg at the knee.
The advantage of rapid backpedaling in
hunting and scavenging niches is that one can
retreat rapidly while still facing a predator or
dangerous prey, before turning ones back and
allowing attack. It has also been observed that
running backward confuses animals, and thus would
be significant in allowing greater distance for
retreat or repositioning.
Thanks to Owen Lovejoy for access to fossil casts.